The critical molecular events that regulate immune control vs. disease pathogenesis in viral hepatitis remain elusive. However, recent reports suggested that patients with chronic hepatitis displayed elevated concentrations of alarmin (IL-33) and its decoy receptor sST2 in the serum. Their levels were associated with hepatic damage, viral and clinical outcomes in response to antiviral therapy. We hypothesize that IL-33 can directly engage innate and adaptive immune systems in the liver and also induce differentiation of novel group 2 innate lymphoid cells (ILC2). Through these actions, IL-33 can modulate immune responses and protect the liver during the inflammatory process. Two complementary approaches are proposed to test these hypotheses. 1) To examine if IL-33 protects the liver in acute hepatitis and persistent viral infection. In this Aim, we will examine he endogenous levels of IL-33 and its receptor ST2 in the liver and serum following i.v. delivery of recombinant adenovirus (rAd). To investigate the IL-33/ST2 pathway in persistent viral infection, we will also examine hepatitis induced by lymphocytic choriomeningitis virus clone 13 (LCVM CL13) and antiviral immune responses in these animals. The function of IL-33 will be assessed in IL-33-/- mice as well as by anti-ST2 mAb in wild-type (wt) animals. Conversely, we will treat infected animals with rIL-33 to evaluate its protective functions. 2) To analyze IL-33-induced immune mechanisms responsible for enhanced immune responses and ameliorated liver injury. Our working hypothesis is that IL-33 can directly act on T effector cells or through inducing ILC2s in the liver. We will test the possibility that IL-33 not only enhances T cell vigor but also expands its antigenic breadth using a reconstituted LCMV TCR transgenic mouse (P14) model. In preliminary studies, we found that IL-33 can enhance IFN-? but inhibit TNF-? production. We hypothesize that pro-IL-33, an unprocessed nucleoprotein, can act as a transcriptional factor to modulate the effector cytokines expression on T cells. To examine the IL-33 and DNA interaction, we will use a two-step ChIP assay to determine whether IL-33 binds and sequesters NF-?B signal for TNF-? expression. We anticipate that lack of IL-33 in the knockout animals will lead NF-?B signaling and TNF-? dysregulation, leading to exacerbated liver injury. The new evidence from this study may explain clinical observations that damage-associated molecules like IL-33 and its receptor correlates to T responses and clinical prognosis in patients with chronic hepatitis and drug-induced hepatotoxicity. Such findings would be potentially paradigm-shifting. Although this research is exploratory in nature, it has strong potential to be developed to more detailed studies involving future therapeutic candidates and human subjects infected with hepatitis virus.